In order to remove fine particles, organic matter, metals and the like from surfaces of electronic materials such as silicon substrates for semiconductors, glass substrates for liquid crystals, and quartz substrates for photomasks, wet cleaning at a high temperature with a hydrogen peroxide-based concentrated chemical solution, so-called RCA cleaning, has been performed. RCA cleaning is an effective method of removing metals and the like on the surfaces of electronic materials. However, as large amounts of high-concentration acids, alkalis, and hydrogen peroxide are used in RCA cleaning, such chemical solutions are contained in liquid waste that is discharged and there is a high neutralization load or precipitation treatment load in waste liquid treatment, and in addition, a large amount of sludge is produced.
Therefore, water containing dissolved gas, prepared by dissolving a particular gas in ultrapure water and adding a small amount of a chemical if needed, has become used in stead of a solution containing a high concentration of the chemical. When cleaning is performed with the water containing dissolved gas, it is less problematic in terms of residual chemicals present on objects to be cleaned, and an improved cleaning effect is obtained, therefore, the amount of cleaning water used can be reduced.
Examples of such a particular gas conventionally used for water containing dissolved gas as cleaning water for electronic materials include hydrogen gas, oxygen gas, ozone gas, noble gas and carbon dioxide gas. In particular, water containing dissolved hydrogen gas to which an infinitesimal amount of ammonia is added exhibits an excellent effect in removing fine particles when it is used in a cleaning step in which it is used in conjunction with ultrasonic waves (for example, Patent Document 1).
Even in the case where cleaning is performed with water containing dissolved gas, when an object to be cleaned is large in size, the amount of the cleaning water used increases. For example, when water containing dissolved hydrogen gas is used, the amount of hydrogen gas used increases, and therefore, the cost of safety measures increase. For this reason, water containing dissolved gas which is less expensive and exhibits an improved cleaning effect is desired for the purpose of further reducing cleaning cost as well.
[Patent Document 1] Japanese Patent Publication No. 2007-243113